Over the past year, this laboratory has continued to study the regulation of expression of genes in normal and malignant human mammary epithelial cells. We have examined the expression several genes associated with the development of drug resistance in human breast cancer cells. We have continued our investigation on the regulation of transcription of the human mdr1 gene. Transfection of a series of mdr1 promoter constructs fused to a reporter gene as well as DNA-protein binding assays were used to identified DNA sequences (Y-box and InR) involved in the regulation of transcription of the human mdr1 gene. We have also examined the expression of the mdr1 gene and the MRP gene expression, another multidrug resistance gene, in leukemias and solid tumors. These studies demonstrated that in AML, MRP but not mdr1 expression increased in leukemic cells obtained at relapse compared to initial presentation. This is the first evidence that suggests a possible role of MRP gene expression in clinical drug resistance. Our laboratory has also continued studies on the regulation of expression of the human glutathione peroxidase 1 gene, an important enzyme activity which protects cells from toxic peroxides formed as a consequence of drug and carcinogen exposure. Studies from our laboratory have shown that the hgpx1 gene maps to chromosome 3P21 and that this gene is commonly deleted in human lung and renal cancers. The role of this gene as a possible tumor susceptibility gene is currently under investigation. We have also continued studies on the role of P53, mdm2, and waf1/cip-1 in cell cycle regulation in normal, immortalized, and transformed mammary epithelial cell lines. These studies have identified important differences in the regulation of P53, MDM2, and waf1 in different stages of mammary tumor development. This past year we have also initiated projects in gene therapy for breast cancer. We have developed expression vectors containing genes that can produce resistance to alkylating agents and are currently investigating their ability to transfer antineoplastic drug resistance into human and murine bone marrow cells. We have also constructed an recombinant defective adenoviral vector containing the wild type P53 gene(AdP53). We have shown that human breast cancer cells are readily infected by recombinant adenoviral vectors. Infection of human breast cancer cells with AdP53 results in high levels of wt P53 expression and induction of MDM2 and Waf1 RNA and protein. Furthermore, AdP53 infection results in cell death in tumor cells lines expressing a mutant P53 gene but not in tumor cells lines or normal mammary epithelial cells. Additional adenoviral vectors for treatment of breast cancer are currently being investigated.